Not applicable.
Not applicable.
The field of the invention is AC to DC converter systems and more specifically a blocking reactor including three cores for blocking harmonic currents in a nine-phase converter system.
Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch-based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion that, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three-phase and therefore three-phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. With respect to cost, as with most hardware intensive configurations, cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
The most common and available type AC to DC converter is a three-phase rectifier system including six semiconductor switches arranged to form a converter that links three AC input lines to positive and negative DC buses where the voltage on the input lines is spaced by 120 electrical degrees. This type of six-switch converter system exhibits relatively high DC output voltage ripple at a frequency that is six times the AC line frequency. For example, where the line frequency is 60 Hertz, the ripple is typically 360 Hertz. Converters that include six switches are generally referred to as six-pulse rectifiers.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases that are rectified where the AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase currents, harmonics and ripple are reduced appreciably. To rectify nine phase currents the industry most solutions employ three three-phase rectifiers, each of the three rectifiers including six switches arranged to form a bridge between each of three of the AC supply lines and DC rectifier outputs. The outputs can be linked in several different fashions to provide one positive DC bus and one negative DC bus as described in more detail below. Three rectifier configurations that include a total of 18 switches are generally referred to as 18 pulse rectifiers.
As the global standard for AC power distribution is three-phase, a mechanism for converting three-phase current to nine-phase current is necessary prior to rectification via any 18-pulse rectifier. To this end the industry has devised several different three to nine-phase transformer configurations. An exemplary three to nine-phase transformer and rectifier configuration is illustrated in FIG. 1 including a transformer 100, and first, second and third rectifiers 120, 140 and 160, respectively, that link three AC supply lines 122, 124 and 126 to positive and negative DC buses 128 and 180, respectively. Transformer 100 receives three 120 degree phase shifted AC currents IA, IB and IC on input lines 122, 124 and 126 and provides nine AC output currents I1 through I9 on nine AC output lines (not numbered) where the output currents include three currents I4-I6 that are in phase with the input currents, three currents I1-I3 that lag the input currents by 20 degrees and three currents I7-I8 that lead the input currents by 20 degrees.
Currents I1 through I3, currents I4 through I6 and currents I7 through I9 are provided to rectifiers 120, 140 and 160, respectively. The outputs of rectifiers 120, 140 and 160 are linked together in parallel. The rectifier input currents I1-I9 are summed together to produce a primary current IA through IC having reduced harmonics. Because the pre-rectified voltages V1-V3, V4-V6 and V7-V9 are spaced out 20 degrees, their rectified DC voltages fill each other""s valleys and hence produce an 18 times fundamental frequency ripple that is relatively smoother when compared to six-switch configurations.
In theory 18 pulse systems like the one illustrated in FIG. 1 have the advantage that each rectifier needs only include components having a power rating corresponding to one third the overall DC output power rating. Thus, in theory 18-pulse rectifier switches in parallel linked configurations can be one third the size of switches required for six pulse rectifiers.
In reality, however, for two reasons the rectifier components have to be greater than the theoretical one-third rated DC size. First, due to manufacturing limitations, slight magnitude differences occur in most cases among the rectifier input voltages. These slight voltage magnitude differences produce slight DC voltage differences at each of the separate rectifier outputs. For example, DC output voltage variance among rectifier outputs is often within the range of 0 to 2 volts.
Converter systems are typically constructed for very low impedance to provide a stiff voltage source to a load. For this reason the slight differences in DC voltage, although small in most cases, cause the rectifier with highest output DC voltage to carry much more DC load current when compared with the current carried by the other rectifiers.
Second, referring again to FIG. 1, in a typical application the three-phase power source would be linked to many loads like the one illustrated and each of those loads would cause some degree of harmonic distortion on supply lines 122, 124 and 126. As known in the industry, the rectified DC voltage for a single three-phase bridge with pre-existing 5th and 7th harmonics is:                               V                      d            ⁢                          xe2x80x83                        ⁢            c                          =                                            3              ⁢                              3                                                    2              ⁢              π                                ⁢                                    V              1                        ⁡                          (                              1                -                                                      1                    5                                    ⁢                                                            V                      5                                                              V                      1                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      φ                    5                                                  -                                                      1                    7                                    ⁢                                                            V                      7                                                              V                      1                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      φ                    7                                                              )                                                          Eq        .                  xe2x80x83                ⁢        1            
with
VA=V1 sin xcfx89t+V5 sin(5xcfx89t+xcfx865)+V7 sin(7xcfx89t+xcfx867)xe2x80x83xe2x80x83Eq. 2
                              V          B                =                                            V              1                        ⁢                          sin              ⁡                              (                                                      ω                    ⁢                                          xe2x80x83                                        ⁢                    t                                    -                                                            2                      ⁢                      π                                        3                                                  )                                              +                                    V              5                        ⁢                          sin              ⁡                              (                                                      5                    ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                    t                                    +                                      φ                    5                                    +                                                            2                      ⁢                      π                                        3                                                  )                                              +                                    V              7                        ⁢                          sin              ⁡                              (                                                      7                    ⁢                                          xe2x80x83                                        ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                    t                                    +                                      φ                    7                                    -                                                            2                      ⁢                      π                                        3                                                  )                                                                        Eq        .                  xe2x80x83                ⁢        3                                                      V            C                    =                                                    V                1                            ⁢                              sin                ⁡                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      t                                        +                                                                  2                        ⁢                        π                                            3                                                        )                                                      +                                          V                5                            ⁢                              sin                ⁡                                  (                                                            5                      ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      t                                        +                                          φ                      5                                        -                                                                  2                        ⁢                        π                                            3                                                        )                                                      +                                          V                7                            ⁢                              sin                ⁡                                  (                                                            7                      ⁢                                              xe2x80x83                                            ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      t                                        +                                          φ                      7                                        +                                                                  2                        ⁢                        π                                            3                                                        )                                                                    ⁢                  xe2x80x83                                    Eq        .                  xe2x80x83                ⁢        4            
Equation 1 indicates that both the magnitude and angle of the harmonic voltages influence the DC voltage. As obvious from FIG. 1, the rectifier input voltages V1-V3, V4-V6 and V7-V9 are spaced out 20 degrees. Thus the values of the harmonic angles (see Equations 1 through 4) for each rectifier 12, 14 and 16, are changed causing the rectified DC voltages from each rectifier to be different. Thus, the pre-existing harmonics also contribute to current unbalance among different rectifiers.
In order to avoid such unbalance problem, one solution is to connect all three bridges in series, instead of in parallel. Referring again to FIG. 1, this type of configuration would include a link between the lower DC output of rectifier 120 and the upper output of rectifier 140, a link between the lower DC output of rectifier 140 and the upper DC output of rectifier 160 and the DC output buses would include the top and bottom DC output leads of rectifiers 120 and 160, respectively. In this case, to achieve the DC output voltage level provided by the parallel configuration described above, the magnitude of each nine-phase voltage V1-V9 would only have to be one-third that of the parallel configuration. Unfortunately, each rectifier 120, 140 and 160 would have to carry the full rated current and therefore the switching devices therein would have to be full-size and relatively expensive.
Other attempts to solve the unbalance problems have employed inter-phase transformers (IPT) having six separate cores between the rectifiers and the DC output rails in parallel rectifier configurations. Unfortunately, with these configurations, each IPT must carry the full DC current generated by the rectifier linked thereto and therefore each IPT must include an air gap adjustment which means that each IPT would be relatively large. In conversion systems where space is limited such excessive space requirements are impractical.
In addition, when one of the rectifiers is out of service for any reason (e.g., a fault condition occurs), the four IPTs corresponding to the other two bridges automatically go into saturation which nullifies the effect of the IPTs entirely.
Yet other attempts to avoid unbalance problems in parallel rectifier configurations have employed harmonic blocking reactors on the AC side of the rectifiers. For example, some efforts have resulted in configurations including three separate reactors that cancel various (e.g., 5th and 7th) voltage harmonics for a six-phase DC to AC system. Other efforts have taught that harmonics in a nine-phase system can be cancelled by adjusting different turn ratios among windings in each of six separate reactors. An exemplary nine phase AC side reactor configuration is illustrated in FIG. 2. In these cases, advantageously, the reactor cores do not need to carry fundamental flux and do not have the saturation problems associated with IPTS. Unfortunately configurations, like the configuration of FIG. 2, that employ AC side reactors have not proven to be much better than the IPT attempts as each attempt requires six separate cores that render required hardware bulky and relatively expensive.
Another AC side reactor configuration is taught by U.S. Pat. No. 4,204,264 in FIG. 3 and includes two separate three-phase cores for a nine-phase AC to DC system. Here instead of using six separate cores as in FIG. 2, six limbs from the two separate cores are employed. While a better effort, this two-core solution still requires a relatively large amount of material to accommodate a nine-phase converter system. In addition, because three-phase cores are used triple harmonic fluxes cannot circulate within the core and the configuration therefore does not eliminate the triple harmonics. Thus, it would be advantageous to configure an AC side harmonic blocking reactor that requires a reduced set of cores, reduces triple harmonics and for which saturation is not a problem.
The present invention is a reactor for linking a multiphase transformer to a rectifier, where the multiphase transformer generates a multiphase AC output signal comprising a plurality of three phase signals. Each of the three phase signals comprises a first, a second and a third output current of substantially similar magnitude, and each of the first, second, and third output currents are spaced one hundred and twenty degrees apart. The first, second, and third output currents of each of the three phase signals are offset from the first, second, and third output currents of another of the three phase signals by a predetermined angle, respectively. The rectifier is for receiving and rectifies the multiphase AC output signals to provide positive and negative DC bus currents. The reactor comprises first, second and third cores and a plurality of winding subsets, the plurality of winding subsets being equal in number to the plurality of three phase signals. Each winding subset includes at least first, second and third windings linked to the first, second and third outputs of the corresponding three phase signal. The windings are arranged on the cores such that at least a winding segment from each of the plurality of winding subsets is wound about each of the first, second and third cores.
In one embodiment of the invention the transformer generates AC output currents having substantially similar magnitudes on each of nine outputs. The first, second and third output currents spaced one hundred and twenty degrees apart, the fourth, fifth and sixth output currents leading the first, second and third output currents by a predetermined angle, respectively, the seventh, eighth and ninth output currents lag the first, second and third output currents by substantially the predetermined angle, respectively. The rectifier receives and rectifies nine phase AC currents to provide positive and negative DC bus currents. The reactor comprises three cores and three winding subsets. The first winding subset includes at least first, second and third windings linked to the first, second and third outputs. The second winding subset including at least first, second and third windings linked to the fourth, fifth and sixth outputs. The third winding subset including at least first, second and third windings linked to the seventh, eighth and ninth outputs. The windings arranged on the cores such that at least a winding segment from each of the first, second and third winding subsets is wound about each of the first, second and third cores.
The windings of the harmonic reactor of the present invention are preferably sized and dimensioned such that, when receiving AC currents at a fundamental frequency, current passing through each winding generates flux within the core such that the fundamental fluxes through the core cancel. The reactor, however, provides impedance to higher order harmonics, thereby providing a blocking function.
In one embodiment of the invention, the reactor is preferably wound such that each of the first, second and third subset first windings are wound about the first core, the first, second and third subset second windings are wound about the second core and the first, second and third subset third windings are wound about the third core. The ratio of the first subset windings to the second and third subset windings on each core is one to one over two times the cosine of the predetermined angle. For a predetermined angle of twenty degrees, and the ratio of first subset windings to second and third subset windings on each core is substantially 1:0.532:0.532.
In another embodiment of the invention, the reactor can be configured such that the first subset first winding, the second subset second winding and the third subset third winding are wound about the first core, the first subset second winding, the second subset third winding and the third subset first winding are wound about the second core and the first subset third winding, the second subset first winding and the third subset second winding are wound about the third core. Again, the ratio of the first subset windings to the second and third subset windings on each core is one to one over two times the cosine of two times the predetermined angle. Here for a predetermined angle of substantially 20 degrees, the ratio of first subset windings to second and third subset windings on each core is substantially 1:0.6527:0.6527.
The reactor of the present invention can also be configured such that the first winding subset includes a single coil, while the second and third winding subsets include first and second coils. Here the first subset first winding, first coil of the second subset first winding, first coil of the second subset second winding, first coil of the third subset first winding and first coil of the third subset third winding are each wound about the first core. The first subset second winding, second coil of the second subset second winding, first coil of the second subset third winding, second coil of the third subset first winding and first coil of the third subset second winding are wound about the second core. The first subset third winding, second coil of the second subset first winding, second coil of the second subset third winding, second coil of the third subset second winding and second coil of the third subset third winding are wound about the third core.
In this embodiment, the reactor can be configured such that the turns ratios of the first subset first winding to the second subset first winding first coil, second subset second winding first coil, third subset first winding first coil and third subset third winding first coil are two to one over two times the cosine of the phase angle between the current linked to the first subset first winding and the current linked to the respective coil. The turns ratios of the first subset second winding to the second subset second winding second coil, second subset third winding first coil, third subset first winding second coil and third subset second winding first coil are two to one over two times the cosine of the phase angle between the current linked to the first subset second winding and the current linked to the respective coil. The turns ratios of the first subset third winding to the second subset first winding second coil, second subset third winding second coil, third subset second winding second coil and third subset third winding second coil are two to one over two times the cosine of the phase angle between the current linked to the first subset first winding and the current linked to the respective coil. This configuration is dimensioned to cancel fundamental flux in each core.
For a predetermined angle of twenty degrees, the turns ratio of the first subset first winding to second subset first winding first coil and second winding first coil is substantially 2:0.532:0.6527, respectively. The turns ratio of the first subset first winding to third subset first winding first coil and third winding first coil is substantially 2:0.532:0.6527, the turns ratio of the first subset second winding to second subset second winding second coil and third winding first coil is substantially 2:0.532:0.6527, respectively, the turns ratio of the first subset second winding to third subset first winding second coil and second winding first coil is substantially 2:0.532:0.6527, the turns ratio of the first subset third winding to second subset first winding second coil and third winding second coil is substantially 2:0.6527:0.532, respectively, and the turns ratio of the first winding second coil is substantially 2:0.6527:0.532.
The reactor of the present invention can also be configured such that each core forms at least one continuous flux path. The cores can be configured as a single or double window, or to include a first, second and third cores forming first, second and third limbs on a four limb core configuration. In this configuration each limb includes first and second ends wherein, each of the first ends are linked and each of the second ends are linked.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which forma part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.